Advanced signal processors for interference cancellation in baseband receivers

ABSTRACT

A multi-mode receiver includes a channel decomposition module (e.g., a Rake receiver) for separating a received signal into multipath components, an interference selector for selecting interfering paths and subchannels, a synthesizer for synthesizing interference signals from selected sub channel symbol estimates, and an interference canceller for cancelling selected interference in the received signal. At least one of the channel decomposition module, the synthesizer, and the interference canceller are configurable for processing multi-mode signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.11/204,606, entitled “Advanced Signal Processors for InterferenceCancellation in Baseband Receivers,” and filed Aug. 15, 2005; whichclaims priority to (1) U.S. patent application Ser. No. 11/192,763,entitled “Interference Cancellation Within Wireless Transceivers,” filedJul. 29, 2005, and issued as U.S. Pat. No. 7,463,609 and (2) U.S. patentapplication Ser. No. 11/100,935, entitled “Construction of ProjectionOperators for Interference Cancellation,” filed Apr. 7, 2005, andpublished as U.S. Patent Application Publication Number 2005-0180364 A1,which claims priority to (a) U.S. patent application Ser. No.10/773,777, entitled “Systems and Methods for Parallel SignalCancellation,” filed Feb. 6, 2004, and issued as U.S. Pat. No.7,394,879; (b) U.S. patent application Ser. No. 10/686,359, entitled“System and Method for Adjusting Phase,” filed Oct. 15, 2003, and issuedas U.S. Pat. No. 7,068,706; (c) U.S. patent application Ser. No.10/686,829, entitled “Method and Apparatus for Channel AmplitudeEstimation and Interference Vector Construction,” filed Oct. 15, 2003,and issued as U.S. Pat. No. 7,580,448; (d) U.S. patent application Ser.No. 10/294,834, entitled “Construction of an Interference Matrix for aCoded Signal Processing Engine,” filed Nov. 15, 2002, and issued as U.S.Pat. No. 7,200,183; and (e) U.S. patent application Ser. No. 10/247,836,entitled “Serial Cancellation Receiver Design for a Coded SignalProcessing Engine,” filed Sep. 20, 2002, and issued as U.S. Pat. No.7,158,559. The entirety of each of the foregoing patents, patentapplications, and patent application publications is incorporated byreference herein.

BACKGROUND

1. Field of the Invention

The invention generally relates to the field of signal processing forwireless communications. More specifically the invention is related tointerference cancellation in single- and multi-mode communicationsystems.

2. Discussion of the Related Art

In order to efficiently utilize time and frequency in a communicationsystem, multiple-access schemes are used to specify how multiple usersor multiple signals share a specified time and frequency allocation.Spread-spectrum techniques may be used to allow multiple users and/orsignals to share the same frequency band and time intervalsimultaneously. Time division multiple access (TDMA) and frequencydivision multiple access (FDMA) assign unique time or frequency slots tothe user. Code division multiple access (CDMA) assigns a unique code todifferentiate each signal and/or user. The codes are typically designedto have minimal cross-correlation to mitigate interference. However,multipath effects introduce cross correlations between codes and causeCDMA systems to be interference-limited.

Multiple-access coding specified by TDMA, FDMA, or CDMA standardsprovides channelization. In a typical CDMA wireless telephony system, atransmitter may transmit a plurality of signals in the same frequencyband by using a combination of scrambling codes and/or spreading (i.e.,orthogonalizing) codes. For example, each transmitter may be identifiedby a unique scrambling code or scrambling-code offset. For the purposeof the exemplary embodiments of the invention, scrambling may denoteencoding data with a W-CDMA scrambling code or encoding data with shortpseudo-noise (PN) sequences, such as used in CDMA2000 and IS-95 systems.

A single transmitter may transmit a plurality of signals sharing thesame scrambling code, but may distinguish between signals with a uniqueorthogonalizing spreading code. Spreading codes, as used herein, encodethe signal and provide channelization of the signal. In W-CDMA,orthogonal variable spreading factor (OVSF) codes are used to spreaddata for multiple access. CDMA2000 and IS-95 employ Walsh covering codesfor multiple-access spreading.

While CDMA signaling has been useful in efficiently utilizing a giventime-frequency band, multipath and other channel effects cause thesecoded signals to interfere with one another. For example, coded signalsmay interfere due to similarities in codes and consequent correlation.Loss of orthogonality between these signals results in interference,such as co-channel and cross-channel interference. Co-channelinterference may include multipath interference from the sametransmitter, wherein a transmitted signal propagates along multiplepaths that arrive at a receiver at different times. Cross-channelinterference may include interference caused by signal paths originatingfrom other transmitters.

Interference degrades communications by causing a receiver toincorrectly detect received transmissions, thus increasing a receiver'serror floor. Interference may also have other deleterious effects oncommunications. For example, interference may diminish capacity of acommunication system, decrease the region of coverage, and/or decreasedata rates. For these reasons, a reduction in interference can improvereception of selected signals.

Multipath and other forms of interference inherently limit theperformance and capacity of other types of transmission protocols. Forexample, Orthogonal Frequency Division Multiplexing (OFDM) and TimeDivision Multiplexing (TDM) may be interference-limited both in uplinkand downlink communications.

Multi-mode transceivers support more than one transmission protocol. Forexample, a wireless handset may support CDMA, Global Standard for MobileCommunication (GSM), and an OFDM wireless local area network protocol.Furthermore, a wireless handset may support a variety of implementationsof a particular transmission protocol. Since different communicationsystems may employ different parameters for designing multiple-accesschannels, the nature of interference between systems can vary greatly.Thus, a multi-mode transceiver may employ a wide variety ofinterference-mitigation strategies. Alternatively, a multi-modetransceiver may employ a single interference-mitigation techniqueadapted to each mode.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide for interferencecancellation in a multi-mode receiver configured to operate in a varietyof communication systems. An interference cancellation system mayinclude a channel decomposition module, a baseband receiver, and aninterference canceller. The interference canceller may include one ormore signal processing components that are configurable for processingsignals from different communication systems. Furthermore, theinterference canceller may include one or more signal processors thatare not configurable, but rather perform common operations for each ofthe multi-mode signals.

Although there are many benefits and applications with respect toparticular embodiments of the invention, one notable benefit of at leastsome of the preferred embodiments is a higher receivedSignal-to-Interference-and-Noise-Ratio (SINR). Embodiments of theinvention are applicable to a broad range of receivers in which anactual or virtual decomposition of received signals into Rake-likechannels precedes interference cancellation.

In one embodiment of the invention, similar steps are provided forprocessing signals in any of a variety of wireless transceiver chains. Afirst step may include obtaining channel estimates that identify and/ormodel multipath components. For example, time-domain or frequency-domainsignals may be used to identify complex gains (including delays) for aplurality of multi path components. A filter having complex coefficientsmay be provided to synthesize multipath.

An optional step may include extracting transmission-source information(such as source-specific scrambling codes) if transmissions from two ormore sources are received. The channel estimates and initial symbolestimates of the transmitted signals may be used to synthesize at leastone multi path component from at least one source. A canceller (such asa subtractive canceller or projection operator) may process thesynthesized signal to remove one or more interference signals (e.g.,multipaths from one or more sources) from one or more predetermined Rakefingers. The projection canceller may provide an optionalsignal-selection process to produce a linear combination of at least oneinterference-cancelled signal and at least one uncancelled signal, suchas to produce a signal output having an SINR greater than (or at leastequal to) the at least one uncancelled signal. Since the projectioncanceller may be located anywhere within the receiver chain, thesynthesis step is configured in accordance with the location of theprojection canceller.

Embodiments of the invention may be employed in a multi-mode receiverconfigured to process multiple transmission protocols (e.g., CDMA, TDMA,OFDM, etc.). Embodiments disclosed herein may be advantageous inmulti-mode receivers employing multiple variations, or modes, of CDMA(e.g., cdmaOne, cdma2000, 1×RTT, cdma 1×EV-DO, cdma 1×EV-DV, cdma20003×, W-CDMA, Broadband CDMA, Universal Mobile Telephone System (UMTS),and/or GPS). Embodiments disclosed herein may be advantageous to systemsemploying multiple modes of OFDM (e.g., IEEE 802.11 a/g/n, IEEE 802.16,IEEE 802.20, multi-band OFDM, spread-OFDM, MC-CDMA, frequency-hoppedOFDM, and DMT). However, the invention is not intended to be limited tosuch systems.

Receivers and cancellation systems described herein may be employed insubscriber-side devices (e.g., cellular handsets, wireless modems, andconsumer premises equipment) and/or server-side devices (e.g., cellularbase stations, wireless access points, wireless routers, wirelessrelays, and repeaters). Chipsets for subscriber-side and/or server-sidedevices may be configured to perform at least some of the receiverand/or cancellation functionality of the embodiments described herein.

Although particular embodiments are described herein, many variationsand permutations of these embodiments fall within the scope and spiritof the invention. Although some benefits and advantages of the preferredembodiments are mentioned, the scope of the invention is not intended tobe limited to particular benefits, uses, or objectives. Rather,embodiments of the invention are intended to be broadly applicable todifferent wireless technologies, system configurations, networks, andtransmission protocols, some of which are illustrated by way of examplein the figures and in the following description of the preferredembodiments. The detailed description and drawings are merelyillustrative of the invention rather than limiting, the scope of theinvention being defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary receiver embodiment of the invention.

FIG. 2 illustrates an embodiment of the invention configured to cancelinterference at any of various points within a CDMA receiver employing arake receiver.

FIG. 3A shows a transmitter part of a W-CDMA system employing open-looptransmit-diversity.

FIG. 3B shows an alternative receiver embodiment according to one aspectof the invention.

FIG. 3C illustrates a multi-mode receiver equipped to cancelinterference in a signal received by a Rake finger.

FIG. 4 illustrates a multi-mode receiver embodiment of the presentinvention configured for processing Orthogonal Frequency DivisionMultiplexing signals or other signals having a cyclic prefix.

FIG. 5 shows an embodiment of a multi-mode receiver configured to cancelinterference in a Global Standard for Mobile Communication system usingTime Division Multiplexing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and are herein described in detail. It should beunderstood, however, that the exemplary embodiments are not intended tolimit the invention to the particular forms disclosed. Instead, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

FIG. 1 illustrates one of many possible multi-mode receiver embodimentsof the invention that include a channel decomposition module 101, abaseband receiver 102, and an interference canceller 103. The basebandreceiver 102 and the interference canceller 103 are configured toprocess received signals transmitted in accordance with a firsttransmission protocol (i.e., a first mode).

The baseband receiver 102 may be regarded as comprising an upstreambaseband processor 121 and a downstream baseband processor 122. Basebandreceiver components preceding (i.e., upstream from) the interferencecanceller 103 in the receiver chain may be referred to as components ofthe upstream baseband processor 121. Baseband receiver componentsfollowing (i.e., downstream from) the interference canceller 103 may bereferred to as components of the downstream baseband processor 122.

Embodiments of the invention may provide for some flexibility indenoting which signal-processing modules belong to the upstream basebandprocessor 121 and which are components of the downstream basebandprocessor 122. A second downstream baseband processor 104 may optionallybe included. Alternatively, the interference canceller 103 output may becoupled back to the downstream baseband processor 122.

An output from the upstream baseband processor 121 may be coupled to theinterference canceller 103. Alternatively, the interference canceller103 may be coupled between the upstream baseband processor 121 and thedownstream baseband processor 122. Coupling of the interferencecanceller 103 to the baseband receiver 102 may denote whichsignal-processing modules are included in the upstream basebandprocessor 121, and consequently, which are included in the downstreambaseband processor 122. The upstream baseband processor 121 may beconfigured to perform one or more baseband signal processing operationson a received baseband signal for producing an estimated informationsignal. The estimated information signal may include a data-symbolestimate, a code-chip estimate, or a sample estimate depending on wherethe interference canceller 103 is coupled to the baseband receiver 102.

In a multi-mode receiver, the baseband receiver 102 is configurable toprocess signals transmitted according to a second transmission protocol(i.e., a second mode). Thus, the interference canceller 103 may besimilarly configurable for processing received signals according to thesecond transmission protocol. The baseband receiver 102 and theinterference canceller 103 may be configured to process signalscorresponding to additional transmission protocols, or modes.Furthermore, the baseband receiver 102 and the interference canceller103 may be configured to process signals corresponding to multiplevariations, or modes, of a transmission protocol.

In an exemplary embodiment of the invention, the baseband receiver 102may comprise a Rake, an equalizer, and a means to select between theRake and the equalizer to optimize signal processing. Thus, the termmulti-mode may be used to express that at least one alternativesignal-processing technique may be employed by a receiver for receivinga signal.

In an exemplary embodiment, the interference canceller 103 may includean interference selector 111, a synthesizer 112, a channel emulator 113,an upstream baseband processor 114, and a cancellation operator 115. Theinterference selector 111 may be coupled to the receive basebandprocessor and configured for selecting subchannels that are likely tocontribute interference to at least one signal of interest. Theinterference selector 111 may produce at least one selected interferencesymbol. The synthesizer 112 is configured to generate a synthesizedinterference signal from at least one selected interference symbol. Thechannel emulator 113 is configured to produce an estimated interferencesignal from the synthesized interference signal.

The upstream baseband processor 114 may be coupled to the channelemulator 113 and configured to produce an estimated receivedinterference signal from the estimated interference signal.Alternatively, the upstream baseband processor 121 may be reused insteadof employing a separate processor (i.e., the baseband processor 114).Both the received baseband signal and the estimated interference signalmay undergo substantially identical signal-processing operations untilthey encounter the canceller 115. Thus, the particular signal-processingoperations performed by the downstream baseband processor 114 depend onwhere in the baseband receiver 102 the interference cancellation isplaced.

The cancellation operator 115 may comprise a projection operatorconfigured for orthogonally or obliquely projecting a signal in thebaseband receiver onto an interference subspace of the estimatedreceived interference. However, other types of cancellation, such assubtractive cancellation, may be performed. Some embodiments of theinvention may provide for scale-invariant subtraction. Followingcancellation, the interference-cancelled signal, an uncancelled signal,or a combination of the interference-cancelled and uncancelled signalsmay be selected for further processing. Any of various signal-qualitymeasures, such as coherence or SINR, may be used in the selectionprocess. The selected signal may be provided with further processing toproduce hard-decision or soft-decision estimates of the data. Multiplesoft-decision estimates may be combined via any of various combiningtechniques that are well known in the art, including Maximal RatioCombining (MRC), Equal Gain Combining (EGC), Minimum Mean Squared ErrorCombining (MMSEC), Minimum Variance Unbiased Combining (MVUC), andSelection Combining.

Each interference-cancellation system may include an output-signalselection module (not explicitly shown) configured to select between theresulting interference-cancelled signal and the uncanceled signal inputto the interference-cancellation system. In some cases, interferencecancellation may result in a less-desirable signal (e.g., a signalhaving a lower SINR) than the original signal input. Thus, it may beadvantageous to select the signal having the highest SINR rather thanassuming that interference cancellation always yields an improvedsignal. Alternatively, a linear combination of interference-cancelledand uncancelled signals may be produced.

Various components of the interference canceller 103 may be adaptablefor switching between a plurality of transmission protocols. Forexample, the synthesizer 112 may perform signal-processing operationsthat are similar to those performed by a transmitter. Thus, thesynthesizer 112 may be configured to generate signals in accordance withtwo or more transmission protocols. Since different receiver basebandprocessing operations may be performed with respect to differenttransmission protocols, the upstream baseband processor 114 may beconfigurable for performing different signal-processing operations inaccordance with the transmission protocol of the received signals.

The projection operator 115 may be adaptable to different transmissionprotocols. The projection operator 115 processes in-phase (I) andquadrature phase (Q) samples, chips, or symbol vectors and/or matrices.However, the bit widths, chip periods, or vector or matrix sizes mayvary with respect to the types of transmission protocol employed.Furthermore, any other components of the interference canceller 103 maybe configurable to operate with respect to multiple transmissionprotocols.

The channel emulator 113 may track signals detected by the channeldecomposition module 101 that are identified as strong sources and/orstrong multipath components. In embodiments comprising Rake fingers,different Rake fingers may work together to receive a predeterminedsignal of interest. Alternatively, each Rake finger may process its ownsignal of interest. In one embodiment of the invention, the channelemulator 113 may be configured to impart a delay to a synthesized signalin order to synchronize the synthesized signal with another component ofthe received baseband signal.

In an exemplary embodiment of the invention, the baseband receiver 102may comprise a Rake (not shown), an equalizer (not shown), and a means(not shown) for selecting between the Rake and the equalizer to optimizesignal processing. Embodiments described herein may be subject toadaptations and permutations that fall within the spirit and scope ofthe claimed invention. In one embodiment of the invention, interferencecancellation may be performed after Rake and/or equalization processing.In an alternative embodiment, interference cancellation may be providedprior to Rake and/or equalizer processing.

In one embodiment, at least one of a received baseband signal, anequalized received baseband signal, and a Rake-processed receivedbaseband signal may be selected for interference cancellation. Inanother embodiment, at least one of an interference-cancelled signal andan uncancelled signal may be selected for further receiver processingfollowing interference cancellation. In this case, the uncancelledsignal may comprise at least one of an equalizer output and a Rakereceiver output.

FIG. 2 illustrates a method embodiment of the invention configured tocancel interference at any of various points within a CDMA receiver. Thefigure demonstrates baseband processing employed in a CDMA system and isused for illustrative purposes. Those skilled in the art will recognizethat other components, such as radio frequency (RF) processingcomponents in the CDMA transceiver may be included.

A CDMA transmitter (not shown) typically formats data, and then performsWalsh coding (W_(r)), scrambling (P[n]), parallel-to-serial conversion(P/S), and pulse-shape filtering (G) prior to coupling into acommunication channel represented by a channel operator (H) 212. Areceived signal coupled from the communication channel 212 is processedby a receiver pulse-shaping filter (G) 214 and then split into aplurality M of multipath signals from at least one source. A channeldecomposition module, such as a Rake receiver that includes M Rakefingers, is configured to decompose the communication channel into aplurality of channel components (i.e., multipath signals). For any kε{1,2, . . . , K}, where K denotes the number of active subchannels orusers, the outputs a_(k) ¹[n], . . . , a_(k) ^(M)[n] of the Rake fingers1 through M at symbol period n are typically combined in order toincrease the SNR of at least one signal of interest.

In one exemplary embodiment, each of the M Rake fingers includes abaseband receiver comprising a delay compensator (e^(ST) ^(m) )216.1-216.M, a chip-rate sampler (J-) 218.1-218.M, a serial-to-parallelconverter (S/P) 220.1-220.M, a descrambler (P[n]) 222.1-222.M, ademultiplexer (W_(r) ^(H)) 224.1-224.M, an optional channel compensator(H) (not shown), and a traffic Walsh selector (e _(k) ^(H)) 226.1-226.M.The baseband receiver may be considered to include the receiverpulse-shaping filter (G) 214.

During ordinary operation, a Rake finger (e.g., the first Rake finger)typically employs receiver-function blocks 216.1, 218.1, 220.1, 222.1,224.1, and 226.1 to produce a first Rake-finger output a_(k) ¹[n] forWalsh channel k at symbol period n in Rake finger 1. Outputs fromdifferent Rake fingers (e.g., a_(k) ¹[n], . . . , a_(k) ^(M)[n]) aretypically combined in a maximal ratio combiner (not shown), whichcombines sub channel estimates for each multipath. Other types ofoptimal combiners may also be used for combining the outputs of thefingers, combining the outputs of multiple receiver antennas, and/orcombining the outputs of fingers in multiple receiver antennas.

In an exemplary embodiment of the invention, each demultiplexer224.1-224.M may comprise a matrix Walsh operator for simultaneouslydespreading all subchannels, rather than a vector Walsh operator that istypically used for despreading only one user subchannel. Furthermore,embodiments of the invention provide for processing each multipathsignal with additional receiver functions configured to performinterference cancellation. For example, each multipath signal may beprocessed with a plurality of projection operators, such as projectionoperators 240.1-240.M, 242.1-242.M, and 244.1-244.M located at variouspoints along the Rake finger receive chain.

The baseband receiver may be regarded as comprising an upstream basebandprocessor and a downstream baseband processor. Baseband receivercomponents preceding (i.e., upstream from) a particular one of theprojection operators 240.1-240.M, 242.1-242.M, and 244.1-244.M in thereceiver chain may be referred to as components of the upstream basebandprocessor. Baseband receiver components following (i.e., downstreamfrom) a particular one of the projection operators 240.1-240.M,242.1-242.M, and 244.1-244.M in the receiver chain may be referred to ascomponents of the downstream baseband processor.

The projection operators 240.1-240.M, 242.1-242.M, and 244.1-244.Mproduce an interference-cancelled version of the multipath signal. Theinterference-cancelled signal or the uncancelled multipath signal may beinserted back into its corresponding Rake finger at the input to thedownstream baseband processor. It should be noted that modules240.1-240.M, 242.1-242.M, and 244.1-244.M may be replaced by subtractiveinterference cancellation operators. A multipath signal may also becoupled out of its respective Rake finger at any of a plurality oflocations within the Rake finger for constructing the projectionoperators 240.1-240.M, 242.1-242.M, and/or 244.1-244.M.

A plurality of interference selectors 228.1-228.M may be configured toidentify and select one or more interfering (e.g., MAI) subchannels ineach Rake finger's multipath signal. The output of each interferenceselector 228.1-228.M may comprise at least one interfering data symbolcorresponding to at least one interfering sub channel. In an exemplaryembodiment of the invention, interference selectors 228.1-228.M mayreceive the outputs of the projection operators 224.1-224.M,respectively. The projection-operator outputs are set to zero (ordiscarded) if they fail to meet a quality criterion, such as coherenceand/or SINR. In an alternative embodiment, projection-operator outputsmay be combined prior to being compared with a threshold. Thus, theinterference selectors 228.1-228.M may be replaced by a single blockconfigured to perform generalized MRC over subchannels and multipaths.In yet another embodiment, a linear combination of cancelled anduncancelled signals may be produced.

In each multipath, the at least one interfering data symbol isre-modulated 204.1-204.M by its original sub channel (e.g., Walsh code),re-scrambled 206.1-206.M, and parallel-to-serial converted 208.1-208.Mto produce a simulated transmit version of an interference signal in atleast one particular multipath signal. Each synthesized interferencesignal is match-filtered 229.1-229.M prior to being delayed 230.1-230.Mby an amount equal to the channel delay experienced by the interferencesignal when it arrived at its original Rake finger.

The matched filters 229.1-229.M may comprise any interpolating filterthat approximates the combined effects of a transmit filter G andreceiver matched-filter G. An exemplary embodiment may employ a linearinterpolator to approximate composite effects of the transmitter,channel path, and receiver. An exemplary embodiment uses a raised-cosinepulse-shaping filter with the standard-specific roll-off factor for thetransmit/receive filters. For embodiments that employ an equalizer, theinterpolating filter may be expressed by GĤG, where Ĥ denotes theequalizer function. The matched filters 229.1-229.M may be consideredpart of at least one of a set of signal processing operations, includingsynthesis, channel emulation, and a second upstream baseband processingoperation.

After the delay 230.1-230.M, each interference signal is coupled to adifferent Rake finger. For example, selected interference signalsoriginating from the first Rake finger are delayed by τ₁ and coupledinto the M^(th) Rake finger. Similarly, selected interference signalsoriginating from the M^(th) Rake finger are delayed by τ_(M) and coupledinto the first Rake finger. The couplings between the first and M^(th)Rake fingers are shown for illustrative purposes only. One skilled inthe art will recognize that embodiments of the invention are intended tobe configurable with respect to many different coupling schemes betweena plurality of Rake fingers. When processing more than two paths, anadditional combiner block (not shown) may be provided for combining allinterference signals relative to a path of interest. For example, whenprocessing interference signals relative to a first Rake finger, theoutputs of the delays 230.2-230.M may be processed (e.g., combined)before coupling into the first Rake finger. A channel emulator mayinclude the delays 232.1-232.M or the combination of delays 230.1-230.Mand 232.1-232.M.

One or more selected interference signals for the M^(th) Rake finger maybe coupled into projection operator 240.M, 242.M, and/or 244.M. In thecase wherein projection operator 240.M is employed, the interferencesignals are coupled directly into the projection operator 240.Mfollowing delay 230.M. A second upstream baseband processor associatedwith the projection operator 240.M may include matched filter 229.Mand/or delay 230.M. The projection operator 240.M may produce aninterference-cancelled signal by projecting the first multipath signalonto a subspace that is substantially orthogonal to an interferencesubspace determined from the M^(th) Rake finger's selected interferencesignals. An alternative means for cancelling interference from thereceived baseband signal may be configured to perform any of variousinterference cancellation techniques, including variations of theprojection techniques described herein, as well as other cancellationtechniques that are well known in the art.

A decision device 241.M may select either the interference-cancelledsignal, the first multi path signal, or a linear combination thereof, byproviding a comparison of the signals with respect to one or more signalquality measures. For example, the decision device 241.M may select asignal having the highest coherence or SINR. An output from the decisiondevice 241.M is coupled back into the first finger at approximately thesame location that the first multipath signal was diverted out of thefirst finger.

In the case where the projection operator 242.M is employed, theinterference signal is processed by a second upstream baseband processorcomprising a delay compensator 232.M, a chip-rate sampler 234.M, aserial-to-parallel converter 236.M, and a descrambler 238.M prior tobeing coupled into the projection operator 242.M. Similarly, the firstmultipath signal is processed by the delay compensator 216.1, thechip-rate sampler 218.1, the serial-to-parallel converter 220.1, and thedescrambler 222.1 prior to being diverted from the first Rake fingerinto the projection operator 242.M. The projection operator 242.Mproduces an interference-cancelled signal by projecting the processedfirst multipath signal onto a subspace that is substantially orthogonalto an interference subspace determined from the M^(th) Rake finger'sselected interference signals. A decision device 243.M selects betweenthe processed first multipath signal and the interference-cancelledsignal produced by the projection operator 242.M. Alternatively, thedecision device 243.M may produce a linear combination of an uncancelledsignal and an interference-cancelled signal. The decision device 243.Mcouples its selection back into the first Rake finger at thedemultiplexer 224.1.

In the case wherein the projection operator 244.M is employed, theinterference signal is processed by a second upstream baseband processorcomprising delay compensator 232.M, chip-rate sampler 234.M,serial-to-parallel converter 236.M, descrambler 238.M, and ademultiplexer 239.M prior to being coupled into the projection operator244.M. Similarly, the first multipath signal is processed by thedemultiplexer 224.1 prior to being diverted from the first Rake fingerinto the projection operator 244.M. The projection operator 244.Mproduces an interference-cancelled signal by projecting the processedfirst multipath signal onto a subspace that is substantially orthogonalto an interference subspace determined from the M^(th) Rake finger'sselected interference signals.

In order to effectively cancel ISI and other interference, theprojection operator 244.M may be configured to cancel interference froma sequence of symbol vectors, rather than just an instantaneous symbolvector. A decision device 245.M selects between the processed firstmultipath signal, the interference-cancelled signal produced by theprojection operator 244.M, or a linear combination of the two. Thedecision device 245.M couples its selection back into the first Rakefinger at the traffic Walsh channel selector 226.1. Channel compensation(not shown) may be provided prior to combining (not shown) with likeWalsh channels from other finger outputs.

Although FIG. 2 illustrates transceiver-chain functionality with respectto a single source (e.g., a base station), the invention may be adaptedto systems having multiple sources. Furthermore, since many receiveroperations are commutative, the order of receiver operations may includealternative configurations. Selectors 226.1-226.M may be replaced by MRCcombiners of raw signals (i.e., received baseband signals) andprojection-cancelled signals. Various components shown herein may beconsolidated into a single component. Similarly, certain components maybe added and/or removed according to particular transceiver designs andcommunication protocols without departing from the spirit and scope ofthe invention.

Exemplary embodiments of the invention may be configured to processtime-division multiplexed signals, such as in a CDMA EV-DO system. Forexample, the data-formatting block 202 and the Walsh coder 204 may beconfigured to process multiple data streams. The symbol vector is timemultiplexed prior to scrambling 206. EV-DO transmissions typicallycomprise a pilot, a MAC, and at least one traffic symbol sequence. Thepilot is assigned Walsh Code 0, the preamble consists of bi-orthogonallymodulated data, the MAC comprises symbols on a set of Walsh codes oflength 64, and the traffic comprises symbols on all 16-length Walshcodes. Therefore, each projection operator 244.m and decision device245.m may be configured to separately process each EV-DO channel(including traffic, MAC, and pilot channels) to provide separateprojection and selection operations for sample-, chip-, or symbol-levelcancellation.

FIG. 3A shows a transmitter part of a W-CDMA system employing open-looptransmit-diversity (OLTD). An exemplary embodiment of the invention isconfigured to operate in a W-CDMA system that uses Alamouti space-timecoding with two transmit antennas to increase network capacity. However,embodiments described herein may be configured to operate in other typesof multi-antenna systems designed for transmit and/or receive diversity.

A data source 301 provides multiple data symbols to primary andsecondary transmit-diversity systems 331 and 332. Since the Alamoutischeme employs a 2×2 matrix coding technique, each of a pair ofspace-time (ST) encoders 303.1 and 303.2 is provided a pair of datasymbols. Both space-time encoders 303.1 and 303.2 are typically providedwith identical pairs of data symbols. Space-time coded symbols arecoupled into Walsh encoders 304.1 and 304.2, which process all activeuser and common channels except pilot and control channels to produce aplurality of Walsh channels. Pilot-signal modules 305.1 and 305.2provide the Walsh channels with P-CPICH and S-CPICH pilot channels,respectively. Gold-code scramblers 306.1 and 306.2 scramble the Walshand pilot channels. Synchronizers 307.1 and 307.2 time-multiplex asynchronization channel (SCH) and a scrambled primary common controlphysical channel (P-CCPCH). The resulting time-multiplexed signal isadded to signal outputs from the Gold-code scramblers 306.1 and 306.2,which are transmitted into a communication channel after processing bypulse shaping filters 308.1 and 308.2 and RF front-end processors (notshown).

An exemplary channel model is illustrated wherein a primary transmitchannel includes two multipath delays 309.11 and 309.12 and associatedgains 339.11 and 339.12 having values h₁₁ and h₁₂, respectively.Similarly, a diversity channel includes two multipath delays 309.21 and309.22 with associated gains 339.21 and 339.22 having values h₂₁ andh₂₂, respectively.

A receiver shown in FIG. 3B is configured to receive a signal accordingto the exemplary channel model. After RF processing (not shown), theresulting baseband signal is processed by a receiver filter 310 matchedto the transmit filters 308.1 and 308.2. Each multipathcomponent/diversity path may be processed in an associated Rake finger341-344. Each finger 341-344 may include a delay compensator311.1-311.4, a chip-rate sampler 312.1-312.4, a de-scrambler313.1-313.4, a Walsh-Hadamard despreader (such as a fast Walshtransform) 314.1-314.4, an optional space-time decoder 315.1-315.4 (forprocessing diversity paths in the case wherein Alamouti space-time codesare employed), and a Walsh traffic-channel selector 316.1-316.4. Theoutput from each finger 341-344 may be provided with channelcompensation before being processed by an optimal combiner 317. Thecombiner 317 may be configured to perform any of various well-knowncombining techniques, including MRC, EGC, SC, and MMSEC.

FIG. 3C illustrates a receiver equipped to cancel interference in asignal received by a first Rake finger 341. A received baseband signalmay be processed by a receiver matched filter 310 before undergoingchannel decomposition, which separates the received baseband signal intomultipath components that are processed by a plurality of Rake fingers.In this exemplary embodiment, an interference-cancellation module 323.1(such as a projection canceller) is employed by the first finger 341. Inalternative embodiments, interference cancellation may be performed foreach of the plurality of Rake fingers.

A baseband receiver may include delay compensators 311.2-311.4,chip-rate samplers 312.2-312.4, descramblers 313.2-313.4, and FWTs314.2-314.4. The baseband receiver also comprises components 311.1,312.1, 313.1, 314.1, 315.1, and 316.1 of the first finger 341. Thereceived baseband signal is coupled into the interference-cancellationmodule 323.1 that is upstream to Rake finger 341 processing. In thiscase, the receiver matched filter 310 may be regarded as an upstreambaseband processor, whereas the first finger 341 may be regarded adownstream baseband processor.

Signal outputs from the Walsh-Hadamard transforms 314.2-314.4 arecoupled into a plurality of interference-selection blocks (e.g.,MAI-selection blocks 320.2-320.4) configured to identify and selectsubchannels that are likely to contribute multiple-access interferenceto at least one signal of interest. In this particular embodiment,space-time decoding is not performed prior to interference synthesis.The outputs of the interference-selection blocks 320.2-320.4 are used bysynthesizers 331.2, 332.1, and 332.2 to synthesize transmittedinterference signals. Synthesizer 331.2 is functionally similar to theprimary transmit-diversity system 331, and synthesizers 332.1 and 332.2are functionally similar to the secondary transmit-diversity system 332.

Signal outputs from the synthesizers 331.2, 332.1, and 332.2 areprocessed by channel emulators 329.12, 329.21, and 329.22, respectively,which compensate for delay induced by the multi path channel to produceestimated interference signals. A combiner 321 produces a linearcombination of the estimated interference signals. A second upstreambaseband processor includes a receiver filter 322 matched to transmitfilter 308.1 and/or 308.2. The estimated interference signal is filteredby filter 322 before being coupled to the interference-cancellationmodule 323.1 along with the received baseband signal output from filter310. The filters 322 and 310 resemble each other such that upstreambaseband processing operations performed on the estimated interferencesignal and the received baseband signal are substantially identical. Theoutput of decision module 324.1 is coupled to the Rake finger 341 (whichcomprises components 311.1, 312.1, 313.1, 314.1, 315.1, and 316.1). Thecombiner 317 may receive inputs from one or more Rake fingers, such asfinger 341.

In some embodiments of the invention, functional blocks may be combined.Other variations and permutations may be implemented without departingfrom the intended scope and spirit of the invention. Alternativeembodiments may consolidate multiple transmit or receive elements into asingle element. In some embodiments, interfering subchannels may beselected using optimally combined data. In such cases, space-timeencoders may be included in the loop. Furthermore, channel gaincompensation and emulation may be provided in the synthesis loop.

A preferred embodiment of the invention may be configurable for aplurality of different communication formats employed in a multi-modetransceiver. For example, receiver and synthesizer components in aninterference-cancellation system may adapt their signal processingoperations depending on the type of transmission protocol employed. FIG.3C illustrates a case in which W-CDMA is employed. However, appropriatesignal-processing modifications to some of the receiver components(e.g., 313.1-313.4, 314.1-314.4, and 331.2-331.4) may be made to adaptthe signal processing to alternative transmission protocols.

FIG. 4 illustrates how an embodiment of the present invention may beconfigured for a receiver in a system employing guard bands and cyclicprefixes, such as systems conforming to IEEE 802.16, 802.11b, and802.11g standards. The receiver shown in FIG. 4 is configured to receivea transmission from a communication channel 409 and process a receivedbaseband signal with a baseband OFDM receiver. An upstream basebandprocessor includes a receiver pulse-shaping filter 411 matched to atransmitter pulse-shaping filter (not shown). A downstream basebandprocessor includes a cyclic-prefix remover 412 for discarding apredetermined signal interval in each received symbol to mitigate ISI, aserial-to-parallel converter (S/P) 413, a Fast Fourier Transform (FFT)414, and an optional sub channel equalizer (EQ) 415.

An interference selector 416 may employ statistical signal processingtechniques to identify active subchannels and discard data symbolestimates corresponding to poor subcarrier-channel conditions. In apreferred embodiment of the invention, the interference selector 416 maycompare soft-decision symbol estimates for each subchannel to a set ofthresholds including a lower threshold and an upper threshold. Symbolsthat do not fall within the two thresholds may be discarded to ensurethat subchannels of interest are relatively free of the effects offading and interference. In one embodiment, the thresholds may bedetermined via measured channel conditions and/or pilot-signal strength.In another embodiment, subchannels may be selected until a qualitymeasure for interference cancellation is met.

A physical channel estimator, such as a physical channel identificationmodule 430, may be configured to process outputs from the OFDM receiverapparatus 401 for providing an equivalent multipath profile. Forexample, the physical channel identification module 430 may processreceived preamble symbols or other received signals having knowncharacteristics for determining the complex frequency response of thechannel and/or an equivalent multipath profile.

Selected data symbols (and their associated sub channel information) arepassed to a synthesizer 402 configured to synthesize one or moreselected interference signals. The synthesizer 402 essentially mimics abaseband transmitter. However, the synthesizer 402 may employ a channelemulator (not shown) that uses the multipath profile produced by thephysical channel identification module 430 to emulate channel 409distortions in the synthesized interference. An inverse FFT (IFFT) 417and a parallel-to-serial converter (P/S) 418 produce a digital sequencecorresponding to selected data symbols mapped onto predeterminedfrequency subchannels (i.e., subcarriers). A cyclic prefix may be added419 to the digital sequence, which is then processed by a transmitterpulse-shaping filter 420. A second upstream baseband processor includesa receiver pulse-shaping filter 421 for processing the synthesizedinterference. Alternatively, a single interpolating filter (not shown)may be employed instead of separate filters 420 and 421.

In one exemplary embodiment of the invention, an interference signalsynthesized by the synthesizer 402 and the received baseband signal arecoupled to a plurality of receiver blocks (such as receiver block 403)wherein each receiver block is associated with a particular multipathdelay. For example, M receiver blocks may be used for M identifiedstrong multipath components. Each received baseband signal that iscoupled into receiver block 403 is first sampled according to symbolboundaries corresponding to a particular multi path component identifiedby the physical channel identification module 430.

For each identified multipath component, one or more interferingsubchannels may be removed from a signal of interest. A resultinginterference-cancelled signal is demodulated, and demodulated signalsfrom the plurality of receiver blocks may be combined in a coherentcombiner 450. The combiner 450 may employ preamble symbol strengths onthe subcarriers in each finger as a combining criterion.

One or more interference-cancellation modules (such as projectionmodules 441-444) are provided for cancelling interference in thereceived baseband signal. In a first exemplary embodiment, the estimatedinterference and the received baseband signal are processed byprojection module 441. The projection module 441 may optionally providechannel emulation to the estimated interference prior to interferencecancellation.

According to one embodiment of the invention, the received basebandsignal may be sampled with respect to a multi path delay selected by thephysical channel identification module 430. Similarly, the interferencesignal may be provided with channel emulation and sampled with the samedelay as the received baseband signal. The received baseband signal andthe interference signal are then coupled to the projection module 441,which projects the interference substantially out of the receivedbaseband signal. The projection module 441 may select as its output theresulting interference-cancelled signal, the received baseband signal,or a linear combination of the two. The selected output signal isprocessed by a cyclic-prefix remover 432, an S/P module 433, and an FFT434 before being combined with other signals (e.g.,interference-cancelled signals and/or received baseband signals) in thecombiner 450. The cyclic-prefix remover 432, S/P module 433, FFT 434,and combiner 450 may be regarded as downstream baseband processingcomponents.

In an alternative embodiment, the upstream baseband processor and thesecond upstream baseband processor include cyclic-prefix removers 432and 422, respectively. Following cyclic prefix removal, the estimatedinterference and the received baseband signal are coupled to projectionmodule 442. The projection module 442 may optionally provide theestimated interference signal with channel emulation prior tointerference cancellation. The S/P module 433 and the FFT 434 processthe projection module's 442 output (which may comprise either or boththe received baseband signal with its cyclic prefix removed and aninterference-cancelled signal produced by the projection module 442)prior to combining 450. The S/P module 433, the FFT 434, and thecombiner 450 may be regarded as downstream baseband processingcomponents.

In another embodiment, the upstream baseband processor comprises thecyclic-prefix remover 432 and the S/P module 433, and the secondupstream baseband processor comprises the cyclic-prefix remover 422 andthe S/P module 423. The FFT 434, and the combiner 450 may be regarded asdownstream baseband processing components. The projection module 443 mayoptionally provide the interference signal with channel emulation priorto interference cancellation. The projection module's 443 output is thenprocessed by the FFT 434 prior to combining 450.

In yet another embodiment of the invention, an upstream basebandprocessor comprising the cyclic-prefix remover 432, the S/P module 433,and the FFT 434 prior to interference cancellation 444 may process thereceived baseband signal. A baseband receiver comprising thecyclic-prefix remover 422, the S/P module 423, and the FFT 424 mayprocess the estimated interference.

In one embodiment, the output of the interference selector 416 mayoptionally be coupled directly to the projection module 444 instead ofbeing processed by the synthesizer 402 and receiver components 422-424.Such processing may be performed, for example, when the multipath delayis some integer multiple of the chip period oversampling factormultiplied by the inverse of the sampling rate, or equivalently, aninteger multiple of the data-symbol duration divided by the FFT length.Thus, signal-processing parameters (such as the sampling rate and theFFT length) may be adapted in response to measured multipath profiles inorder to simplify the receiver system and/or improve receiverperformance.

Similar simplifications may be used to eliminate components shown hereinwhen cancellation is performed downstream in the receiver chain. Theoutput of the projection module 444 may be sent directly to the combiner450. The output of the combiner 450 is typically followed by additionalbaseband processing modules (not shown), including deinterleaving,despreading, descrambling, and channel decoding.

In an exemplary embodiment of the invention, interference estimates froma first multipath signal may be used to cancel interference in areceived baseband signal processed with respect to a second multipathsignal. For example, a projection operator configured to project outinterference in a first multipath component of a received basebandsignal may use interference estimates determined from one or moremultipath components other than the first component.

Embodiments of the invention may be useful for reducing the cyclicprefix or guard interval used in multi carrier systems. Some embodimentsmay be used for mitigating inter-channel interference due to Dopplershifts in Doppler-spread channels. Receiver embodiments may also findutility in exploiting transmitted energy in the cyclic prefix of aconventional multicarrier signal. While symbol-level cancellation iswell known for mitigating ISI in an OFDM system, some embodiments of thepresent invention provide for chip- or sample-level cancellation. Theterm “sample level” denotes that multiple time-domain samples per chipare processed in a canceller.

FIG. 5 shows an embodiment of the invention adapted to cancelinterference in a receiver configured to process signals in a GSMsystem. In GSM, co-channel interference arises when adjacent basestations use the same frequency subchannel to transmit to subscribers.In an exemplary embodiment, transmitted data symbols a₁ and a₂ intendedfor different users are transmitted in the same subchannel by adjacentbase stations. The transmissions corresponding to a₁ and a₂ undergodifferent channel effects, which are represented by channel blocks H₁and H₂, respectively.

Signals received by an interference receiver 501 are down-converted intoreceived baseband signals. The interference receiver 501 is configuredto select one or more interference signals that may interfere with atleast one signal of interest. In this case, symbol a₁ is a symbol ofinterest and symbol a₂ is an interfering symbol. The interferencereceiver 501 demodulates the selected interference symbol H₂ a₂, whichis coupled to a transmission synthesizer 502.

The interference receiver 501 may be configured to determine channelinformation from known training sequences in the transmission. Forexample, mid-ambles (which are typically used in GSM transmissions) maybe correlated with training sequence codes (TSC) to obtain channelinformation. A GSM user may be assigned one of eight possible TSCs.

In one embodiment of the invention, the interference receiver 501 maycomprise an advanced receiver capable of suppressing multipathinterference. The receivers 501 and/or 503 may optionally exploitspectral inefficiencies of transmissions employing real constellations(e.g., GMSK, PAM, BPSK) by combining the real and imaginary channels,each of which is modulated with the same real symbol. Methods andarchitectures for providing first-order estimates of the signal are wellknown in the art and may be used for interference detection as well. Forexample, one may use equalizers and pre-whitening filters for accuratefirst-order estimates. However, the complexity of such filters may bereduced if only a coarse first-order estimate is required.

The transmission synthesizer 502 GMSK modulates and pulse shapes theselected interference. The transmission synthesizer 502 may optionallyinclude a channel emulator (not shown). Synthesized signals produced bythe transmission synthesizer 502, as well as the received basebandsignal, are coupled into a cancellation receiver 503. The cancellationreceiver 503 may be configured to orthogonally or obliquely project thereceived baseband signal onto a subspace corresponding to the selectedinterference. Either an interference-cancelled signal produced by thecancellation receiver 503 or the uncancelled signal (i.e., the receivedbaseband signal) may be selected based on measured SINR or some othersignal-quality parameter. Alternatively, the cancellation receiver 503may output a linear combination of the interference-cancelled signal andthe uncancelled signal. The projection canceller may be replaced by asubtractive canceller.

The method and system embodiments described herein merely illustrateparticular embodiments of the invention. It should be appreciated thatthose skilled in the art will be able to devise various arrangements,which, although not explicitly described or shown herein, embody theprinciples of the invention and are included within its spirit andscope. Furthermore, all examples and conditional language recited hereinare intended to be only for pedagogical purposes to aid the reader inunderstanding the principles of the invention. This disclosure and itsassociated references are to be construed as being without limitation tosuch specifically recited examples and conditions. Moreover, allstatements herein reciting principles, aspects, and embodiments of theinvention, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

It should be appreciated by those skilled in the art that the blockdiagrams herein represent conceptual views of illustrative circuitry,algorithms, and functional steps embodying principles of the invention.Similarly, it should be appreciated that any flow charts, flow diagrams,system diagrams, and the like represent various processes which may besubstantially represented in computer-readable medium, includingcomputer-readable medium storing a computer program, and so executed bya computer or processor, whether or not such computer or processor isexplicitly shown.

The functions of the various elements shown in the drawings, includingfunctional blocks labeled as “processors” or “systems,” may be providedthrough the use of dedicated hardware as well as hardware capable ofexecuting software in association with appropriate software. Whenprovided by a processor, the functions may be provided by a singlededicated processor, by a shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” should not be construed to refer exclusivelyto hardware capable of executing software, and may implicitly include,without limitation, digital signal processor (DSP) hardware, read-onlymemory (ROM) for storing software, random access memory (RAM), andnonvolatile storage. Other hardware, conventional and/or custom, mayalso be included. Similarly, the function of any component or devicedescribed herein may be carried out through the operation of programlogic, through dedicated logic, through the interaction of programcontrol and dedicated logic, or even manually, the particular techniquebeing selectable by the implementer as more specifically understood fromthe context.

Any element expressed herein as a means for performing a specifiedfunction is intended to encompass any way of performing that functionincluding, for example, a combination of circuit elements which performsthat function or software in any form, including, therefore, firmware,micro-code or the like, combined with appropriate circuitry forexecuting that software to perform the function. Embodiments of theinvention as described herein reside in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the operational descriptions callfor. Applicant regards any means which can provide those functionalitiesas equivalent as those shown herein.

The invention claimed is:
 1. A cancellation method comprising: providinga received signal that is decomposable into one or more multiple-accessinterference (“MAI”)-channel signals; applying a weight to each of theone or more MAI-channel signals to produce one or more weightedMAI-channel signals; and subtracting one or more of the weightedMAI-channel signals from the received signal to create aninterference-canceled signal.
 2. The method of claim 1, wherein a firstweighted MAI-channel signal is coupled into a first stage of a firstRake finger.
 3. The method of claim 1, wherein applying a weightcomprises combining the one or more weighted MAI-channel signals toproduce at least one of an interference matrix and an interferencevector.
 4. The method of claim 1, wherein applying a weight comprises atleast one of determining complex weights of each of the one or moreMAI-channel signals or determining estimation errors for each of thecomplex weights.
 5. The method of claim 1, wherein providing a receivedsignal comprises a multi-antenna operation comprising diversitycombining and beam forming.
 6. The method of claim 1, wherein applying aweight comprises producing a linear combination of the one or moreweighted MAI-channel signals.
 7. A system for interference cancellation,comprising: a receiver configured to decompose a received signal into aplurality of signal paths, including one or more multiple-accessinterference (“MAI”)-channel paths; a weighted decision combinerconfigured to apply a weight to each of the one or more MAI-channelpaths to produce one or more weighted MAI-channel signals; and acancellation operator configured to subtract one or more of the weightedMAI-channel signals from the received signal to create aninterference-canceled signal.
 8. The system of claim 7, furthercomprising a gain correction module configured to compensate for gainapplied to one or more of the plurality of signal paths by one or moretransmitters.
 9. The system of claim 7, wherein the receiver isconfigured to receive signals from a transmit diversity system.
 10. Thesystem of claim 9, wherein the receiver is configured to receive signalsfrom at least one of a space-time transmit diversity (STTD) system and aclosed loop transmit diversity system.
 11. The system of claim 7,wherein the receiver comprises a Rake receiver.
 12. The system of claim11, wherein the Rake receiver includes at least one multi-antennareceiver configured to provide at least one of diversity combining andbeam forming.
 13. The system of claim 11, further comprising a delayelement coupled between the Rake receiver and the cancellation operatorand configured to impart a delay to at least one of the plurality ofsignal paths.
 14. The system of claim 11, wherein the Rake receiverincludes at least one of a pulse-shaping filter, a combiner, and asearcher/tracker module.
 15. The system of claim 11, wherein the Rakereceiver and the cancellation operator are configured with an iterativefeedback loop.
 16. The system of claim 7, wherein the weighted decisioncombiner and the cancellation operator are coupled between at least oneof a pair of system components, including a sampler and a descrambler, achannel compensator and a descrambler, a descrambler and ademultiplexer, or a demultiplexer and a gain correction module.
 17. Thesystem of claim 7, further comprising a gain correction moduleconfigured to compensate for gain applied to one or more of theplurality of signal paths by one or more transmitters.
 18. A system forinterference cancellation, comprising: a receiver configured todecompose a received baseband signal into a plurality of channels; oneor more interference selectors configured to select one or more of theplurality of channels that are likely to contribute multiple-accessinterference (“MAI”) to at least one signal of interest; one or morechannel emulators configured to apply one or more complex gains to theselected one or more channels to produce one or more scaled MAI-channelsignals; and one or more cancellation operators configured to subtractone or more of the scaled MAI-channel signals from the received signalto produce an interference canceled signal.
 19. The system of claim 18,further comprising one or more baseband signal reconstruction modulesconfigured to process the selected one or more of the plurality ofchannels to produce one or more processed signals that are substantiallyin the same form as the received baseband signal.
 20. The system ofclaim 18, further comprising a gain correction module configured tocompensate for gain applied to one or more of the plurality of channelsby one or more transmitters.
 21. The system of claim 18, wherein thereceiver comprises a Rake receiver.
 22. The system of claim 18, whereinthe one or more channel emulators and the one or more cancellationoperators are coupled between at least one of a pair of systemcomponents, including a sampler and a descrambler, a channel compensatorand a descrambler, a descrambler and a demultiplexer, and ademultiplexer and a gain correction module.
 23. A system forinterference cancellation comprising: a receiver configured to decomposea received signal into a plurality of channels; a sampler configured tosample the received signal; a channel estimator coupled to the samplerand configured to produce one or more complex channel estimates; and acancellation operator configured to subtract one or more of the complexchannel estimates from the received signal to create aninterference-canceled signal.
 24. The system of claim 23, furthercomprising a matched baseband filter coupled to the sampler andconfigured to filter the one or more of the plurality of channels. 25.The system of claim 23, further comprising a channel compensatorconfigured to produce one or more channel-compensated signals.
 26. Thesystem of claim 25, wherein the channel compensator is configured toproduce one or more channel-compensated signals that compensates fortransmit antenna weights in addition to channel effects.
 27. The systemof claim 23, wherein the channel estimator is configured to receiveprimary common pilot channel (P-CPICH) data bits, the P-CPICH OVSF code,and the sampled signal from the sampler.
 28. The system of claim 23,wherein the channel estimator is configured to receive secondary commonpilot channel (S-CPICH) data bits, the S-CPICH OVSF code, and thesampled signal from the sampler.
 29. The system of claim 23, furthercomprising a gain correction module configured to compensate for gainapplied to one or more of the plurality of channels by one or moretransmitters.